Americans who think global warming is caused by human activity, including vehicle and industrial emissions, are now a minority for the first time in nearly two years, a CNN/Opinion Research Corporation poll said on Monday.

Only 45 per cent of the 1,041 adults surveyed on December 2-3 believed global warming is a proven fact and mostly caused by human activity, down from 56 per cent in October 2007, the survey found.

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By comparison, Americans who believe global warming is caused by natural changes unrelated to man have increased to 22 per cent from 20 per cent two years ago, and those who believe global warming is a yet unproven theory grew to 31 per cent from 23 per cent.

Regarding how the United States should tackle global warming, 58 per cent of those surveyed said it should cut carbon dioxide emissions unilaterally, down from 66 per cent in October 2007.